1. Field of the Invention
The present invention relates to the preparation of a sheet of plastic, in particular those intended for applications in which excellent transparency, or other optical properties, are required. This is the case, for example, with glazings in buildings or transportation vehicles, which could conceivably be replaced, at least in order to meet particular requirements, by sheets of plastic. The invention also relates to plastic sheets and glazings containing plastic sheets.
2. Discussion of the Background
Compared to glass, plastics are lighter--something which is a key advantage in urban vehicles powered by an electric motor insofar as an increase in their range is crucial. In such vehicles, it would even be conceivable to produce entire doors, or even whole sides of the bodywork, including the windows, from plastic, as a single unit, and then paint the bottom half, if desired. In general, making transparent surfaces lightweight is useful with regard to modern transportation vehicles insofar as technical progress goes hand in hand with incorporating ever more numerous functions into the glazing itself (rear-window heating, radio antenna, windscreen defrosting, tinting in order to prevent the passenger compartment from heating up in strong sunlight, incorporation of electrochromic compounds, display of information on the windscreen, etc.), and with using ever greater areas of glazing. This results in an increase in the overall weight of the vehicle, to the detriment of its energy consumption.
Moreover, compared to glass, plastics are capable of providing improved safety features and increased protection against theft, because of their superior toughness. A no less important advantage that plastics have over glass is their greater ability to be easily converted into complex shapes. Finally, the ability of sheets of plastic to be reversibly deformed to a greater or lesser extent makes it possible to envisage considerably simplified ways of fitting them into the bodywork openings, by means of snap-fastening, from the inside or from the outside of the vehicle.
In contrast, compared to glass, its relatively low rigidity mentioned above is obviously a handicap, as are its inferior transparency and optical properties and, above all, its greater scratchability.
Several approaches have hitherto been adopted in order to try to overcome this triple handicap.
According to a first approach, flat sheets of plastic are formed by extrusion, a piece is cut to the required dimensions and fixed to a thermoforming device, the thermoforming is carried out by contact with at least one solid mould surface and, optionally, using compressed air or suction. The optical properties of a sheet thus extruded are not satisfactory.
According to a second approach, the plastic is injection molded directly in a mold of suitable shape. The complexity of the manufacture of injection-molding devices is proportional to that of the shapes which it is desired to obtain; the cost of these devices feels the effect of this. In addition, this technique lacks flexibility in the sense that each component shape to be obtained requires the use of a special injection molding device. What is more, when substantially non-planar shapes, shapes which are more than just slightly curved, or which are complex in some other way are required, a non-uniform distribution of the material in the mold is observed, even when strictly conforming with the optimum operating conditions, such as the temperature/viscosity and pressure. This non-uniformity adversely affects the optical properties of the product.
According to both the above approaches, shaping is carried out by contact with a solid surface, generally made of metal. Of course, any imperfections in this solid surface are reproduced in the shaped plastic. Consequently, achieving good optical properties relies on an operation to polish the surface of the mold and/or of the molded component.
Moreover, the abovementioned scratchability of plastics is such that, in their optical application or as transparent elements, it is necessary to coat the shaped components with a hard varnish. This operation is accompanied, as is well-known to those skilled in the art, by problems of the varnish flaking off, problems which are all the more acute in the case of surfaces having complex shapes. In addition, it has only been envisaged to form the hard varnish at a temperature below the distortion temperature or softening point of the plastic, the shape of which is thus completely preserved throughout this operation. Such conditions for forming the varnish are excessively restrictive and have resulted in considerable effort being expended on producing varnishes which form at sufficiently low temperatures and, at the same time, producing thermoplastics having high softening points.
There is therefore still a need for a process for obtaining plastic components, in particular ones which are transparent or intended for optical applications, which would avoid the poor optical quality inherent in extrusion techniques or due to shaping components by contact with imperfect solid surfaces, to the non-uniform distribution of material in the injection mold, and to the problem of varnish flaking, and which would allow numerous varnishes to be used in combination with numerous plastics under satisfactory conditions of compatibility.